The present invention relates generally to electric machines, and more particularly to doubly salient machines having stator teeth with angled permanent magnets therein.
A variety of permanent magnet machines, including doubly salient machines, are known in the art in which permanent magnets are positioned within the frame or back iron of a stator. This is typically done to accommodate relatively large magnets capable of producing significant flux. As recognized by the inventor, however, positioning magnets in the stator frame tends to weaken the stator structure, including along those portions where the stator teeth meet the stator frame, and often leads to acoustic noise problems. In many designs, permanent magnets are associated with each stator pole, often with an alternating polarity for each adjacent stator tooth. As recognized by the inventor, however, such a configuration requires an excessive number of permanent magnets as well as a relatively complex device for magnetizing the stator magnets, thus increasing the complexity and manufacturing cost of the permanent magnet machine.
In order to solve these and other needs in the art, the inventor hereof has succeeded at designing and developing permanent magnet machines, including doubly salient machines, having one or more permanent magnets located at least partly and preferably entirely within the stator teeth, thereby avoiding weakening of the stator back iron structure while reducing acoustic noise. In one embodiment, permanent magnets are located in only a subset of the stator teeth, thereby lowering magnet material and manufacturing costs. In another embodiment, all such magnets are oriented with their north poles facing inwardly (i.e., toward an interior of the machine), resulting in reduced cogging and negative torques with an improved torque density. The permanent magnets may also extend within the stator teeth on an angle or diagonal, thereby allowing use of magnets which are wider than the teeth themselves to produce greater flux. Further, a magnetizing device having a single coil may be used to readily and simultaneously magnetize all the stator magnets with a common polarity.
In accordance with one aspect of the present invention, a stator for use in a permanent magnet machine includes a frame having an outer peripheral edge and an inner peripheral edge extending about a central axis, a plurality of stator teeth each extending along a radial axis from the frame""s inner peripheral edge toward the central axis, and at least one permanent magnet located at least partly within one of the stator teeth, wherein said one permanent magnet is oriented at an oblique angle relative to the radial axis along which said one of the stator teeth extends.
In accordance with another aspect of the invention, a stator for use in a permanent magnet machine includes a frame having an outer peripheral edge and an inner peripheral edge extending about a central axis, a plurality of permanent magnets each having inwardly facing north poles, a first plurality of stator teeth each extending along a radial axis from the frame""s inner peripheral edge toward the central axis, and a second plurality of stator teeth extending from the frame""s inner peripheral edge toward the central axis. Each of the first plurality of stator teeth has one of the permanent magnets located at least partly therein, while the second plurality of stator teeth each have no permanent magnets located therein. The first plurality of stator teeth are each positioned directly between two of the second plurality of stator teeth. The north poles of the permanent magnets are each oriented at an oblique angle relative to the radial axis along which its corresponding one of the stator teeth extends. Each permanent magnet and its corresponding one of the stator teeth has a width extending in a direction of rotation of a rotor when the rotor is mounted for rotation about the central axis. The width of each permanent magnet is greater than the width of its corresponding one of the stator teeth.
In accordance with a further aspect of the invention, a permanent magnet machine includes a stator of the type described herein.
In accordance with yet another aspect of the invention, a method is provided for magnetizing a stator of an electric machine having a plurality of stator teeth spaced about a central axis, and a plurality of magnets, using a magnetizing device which includes a post and a coil extending around a central axis. The method includes positioning the stator relative to the magnetizing device with the central axis of the stator generally parallel to the central axis of the coil, and with the post extending adjacent the plurality of stator teeth, and energizing the coil to induce flux through the post, through at least some of the stator teeth, and through at least some of the magnets to thereby magnetize at least some of the magnets.